28
c) Use the appropriate Position switch (16) or
(17) to move the trace to the centre of the
screen graticule (= the horizontal centre scale
line).
d) The POS switch must not be moved again
until the measurement has been completed.
e) Then set the AC/GND/DC switch to DC,
making absolutely sure that the maximum
input values are not exceeded. All signals
above the centre line are positive and all
those below it are negative.
f) Determine the interval between the positive
or negative line and the centre line by count-
ing the scale marks or the "boxes" = DIVI-
SIONS and multiply this number by the setting
of the VOLTS/DIV switch.
Example: The interval is 2.8 DIVs (= 2 boxes
and 4 scale marks x 0.2 cm). The VOLTS/DIV
switch is set to 2 VOLTS/DIV. This gives 2.8
DIVs x 2 VOLTS/DIV = 5.6 V.
g) If X5 expansion has been set (by pulling out
the VARIABLE knob), you must divide the
determined value by 5, if a 1:10 probe is
connected, you must multiply the determined
value by 10.
2.3.2 Measurement of time, time intervals
To perform this type of measurement, proceed
as follows:
1. Set up the oscilloscope according to 2.2.4.
2. Then set the TIME/DIV switch so that the
signal image (a half-wave) is displayed on the
screen as large as possible.
3. Use the Vertical and Horizontal position
switches [POSITION (16) or (17) and (25)] to
move the image so that the start of the time
interval coincides with a vertical graticule line
and its end is on the horizontal centre line of
the graticule.
4. The time is then determined as follows:
Time = interval between start point and end
point of half-wave in DIVs x setting of the
TIME/DIV switch.
Example: The interval is 5.4 DIVs and the
TIME/DIV switch is set to 5 µs/DIV. This gives
5.4 DIV x 5 µs/DIV = 27 µs. The time is 27 µs
(microseconds = 10
-6
seconds).
2.3.3 Measurement of period, pulse width etc.
This measurement is performed in a similar way
to that described under heading 2.3.2.
The period of a square-wave pulse or any other
waveform is the time taken to complete a full
cycle (360º) (see also figure).
Fig. 16
In this figure, the time-base sweep is 10 ms/DIV;
the measurement is made between A and C,
thus giving a period of 70 ms.
The pulse width is the distance between A and
B, i.e. roughly 1.5 div which is equivalent to 15
ms. The larger the waveform displayed, the
more accurate the reading or measurement
result. To achieve this, either select the next
smallest time-base sweep (to the left) or pull out
the VARIABLE knob (24), but in the calibrated
position (right-hand stop). The X position knob
(25) can then be used to position the signal more
accurately.
Once the pulse width and the period are known,
the mark-to-space ratio can be determined as
follows:
Pulse width x 100 15 ms x 100
= ——————— = ————— = 21,4 %
Period 70 ms
2.3.4 Frequency measurements
Wherever possible, a frequency counter should
be used to obtain an accurate frequency indi-
cation. The counter can be connected to the
channel 1 output at the rear of the oscilloscope.
However, the oscilloscope can be used to de-
termine a frequency if a counter is not available
or in the case of modulated signals or noise
where the counter does not respond.
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